>>30296106because water moves gain additional damage in the rain and weakens fire moves, the rain also makes thunder 100% accurate, gives bonuses depending on the ability (rain dish, swift swim, hydration), reduces the recovery of weather-dependent recovery moves like sunlight and moonlight which really hurts mons like arcanine, its duration can be extended with damp rock, and there are mons that passively cause rain to appear just by switching in
meanwhile trick room just makes the slower mon move first. there are no other bonuses and you can't extend it's duration
